Explain energy balance and basal metabolic rate, list the factors affecting energy needs, and predict the effect of an energy surplus or deficit

What this dot point is asking

The syllabus wants you to explain energy balance, define basal metabolic rate, list what changes a person's energy needs, and predict the effect on weight when intake and output do not match. The central idea is simple but powerful: weight stays steady only when energy in equals energy out, and any lasting mismatch changes body weight.

The answer

Energy in and energy out

The body takes in energy from the food it digests (mostly from carbohydrate and fat) and uses energy for everything it does: staying alive, moving, and keeping warm. Energy is measured in kilojoules (kJ) or kilocalories (kcal).

Energy balance

Energy balance is the state in which energy intake equals energy output, so body weight stays constant. There are three possible states:

• Balance: energy in $=$ energy out, weight stays the same.
• Surplus: energy in $>$ energy out, the extra is stored as body fat and weight increases.
• Deficit: energy in $<$ energy out, the body uses its fat stores and weight decreases.

Basal metabolic rate (BMR)

Basal metabolic rate is the energy the body uses at complete rest just to stay alive, for example to keep the heart beating, the lungs working, the organs functioning and the body warm. BMR is the largest part of most people's total energy use, even before any activity is added.

Factors that affect energy needs

• Age: children and teenagers need extra energy for growth and activity; needs usually fall in older age.
• Physical activity: an active person or a manual worker uses far more energy than someone sitting most of the day.
• Body size and composition: larger people, and people with more muscle, have higher BMR and higher needs.
• Gender: on average, males have a higher BMR and higher energy needs than females of the same age.
• Pregnancy and breastfeeding: raise a woman's energy needs.
• Climate and illness: the body may use more energy to stay warm or to recover.

Why this matters

If energy intake stays above output, the surplus is stored as fat and the person gains weight, risking obesity and its related diseases. If intake stays below output, the person loses weight, which is the basis of healthy weight loss but, if extreme, can lead to being underweight and malnourished.

Examples in context

Example 1. A teenager during exam season. Sitting and studying for long hours lowers activity, so energy output falls. If the teenager keeps snacking at the same rate, intake now exceeds output, creating a surplus that is stored as fat. Matching intake to the lower activity, or adding exercise breaks, restores balance.

Example 2. A manual worker versus an office worker. A construction worker in Singapore's heat uses far more energy through activity than someone at a desk, so the worker needs a larger energy intake to stay in balance. The same large meal that maintains the worker's weight could cause weight gain in the office worker.

Try this

• Cue. Define basal metabolic rate and explain why it is part of total energy needs. State that BMR is the energy used at rest to stay alive, and total needs add activity on top.
• Cue. A person eats $9500\ \text{kJ}$ and uses $11000\ \text{kJ}$ a day. State the balance and the effect on weight. Work out a $1500\ \text{kJ}$ deficit, so the body uses fat stores and weight falls.
• Cue. List three factors that raise a person's energy needs. Recall higher activity, growth in children and teenagers, larger body size, or pregnancy.